Partial discharge monitoring system for transformers

ABSTRACT

An apparatus for detecting partial discharge in on-line high voltage electrical equipment containing a dielectric, such as high voltage transformers. The apparatus includes an ultrasonic transducer and an annular capacitive plate for detecting, respectively, the ultrasonic pulse and the radio frequency pulse generated by the occurrence of partial discharge in the equipment. The outputs of the transducers are analyzed by signal processing means to allow determination of occurrences of true partial discharge from the plethora of electrical noise normally present in on-line voltage equipment

TECHNICAL FIELD

The present invention relates generally to a system for monitoring theperformance of electrical equipment, such as high voltage transformers.In particular, the system can detect the occurrence of faults in theoverall insulation of such transformers and provide signals that triggerlocal and/or remote alarms indicative of the faults.

BACKGROUND ART

High voltage generator and transmission transformers form an integralpart of any electrical power generation distribution and transmissionsystem. Other transformers, such as rectifier transformers are, alsoused in industrial processes, such as smelting electro-depositionprocesses. Also, current transformers (CTs) are used for protection andmetering of electricity distribution systems.

The most important part of the insulation for oil filled transformerscomprises paper which is wound around the copper windings. There arespacers, washers, seals, lead through plates, taps and bushings, whichare also part of the insulation system within the transformer. In orderto enhance the insulation and stability, the paper is permeated with adielectric, typically mineral oil or silicone oil, which fills thetransformer. This insulating oil also serves as a coolant, distributingheat by convection or forced flow, and also quenches discharges. Othertypes of transformers include high frequency communication transformerswhich use solid polymeric dielectrics such as epoxy thermoset, which isvacuum back-filled into the transformer, and gas-filled transformers.Gas-filled transformers, for example those used in underground mines,are usually filled with argon or sulfur hexafluoride for safety. Thereare also some low voltage air filled transformers.

The operating lifetime of a high voltage transformer can be greater than35 years. The lifetime depends on the loading, design, quality ofmanufacture, and materials and maintenance routines. During itslifetime, the transformer insulation can degrade, the rate ofdegradation being dependent upon the workload and the internal operatingenvironment of the transformer, such as temperature, moisture content,pH and the like. Any degradation of the insulation, such as electronicand ionic plasma erosion of solid insulation surrounding an air bubbleoccluded due to faulty manufacture, can result in increasing levels ofpartial discharge within the transformer. Occurrence of partialdischarges also leads to evolution of gases such as hydrogen andacetylene within the transformer. Such increased partial discharge leadsto further degradation of the insulation which in turn leads toincreasable levels of partial discharge. Continued degradation of theinsulation can result in severe discharges, short-circuit faults or acatastrophic failure due to an explosion of the gases, for example,hydrogen, acetylene and ethylene, produced as chemical by-products ofthe degradation process. Such failure can result in reduction or loss ofsupply to the power system, incur considerable expense for thereplacement or repair of the transformer and also present a serious riskto nearby personnel and the environment.

Partial discharge in transformers can also occur due to faultymanufacture and/or mechanical or electrical fatigue. For example, themovement of loose components, and creep and stress relaxation ofmetallic components, such as fastenings, or foreign metallic bodieswithin the transformer, provide an opportunity for discharges to occureven when there has been no or little degradation of the insulation.

Partial discharge in transformers can also arise due to windingsbecoming loose within the transformer. Wear and tear suffered by the tapconnectors in the tap changer can also cause partial discharges. Faultsin the bushings can also result in partial discharges.

It is known that a partial discharge can produce signals at differentlocations within a large transformer including a discharge current inneutral caused by imbalance, a displacement current through thecapacitive tapping of a bushing, a radiated radio frequency (RF) pulseor wave and a radiated ultrasonic (US) pulse or wave.

The magnitude of partial discharge within a transformer provides onemeans of determining the integrity of the transformer's insulation. Forexample, a detected partial discharge having a magnitude of 50 pC wouldnormally be ignored at normal voltage operations, a reading of 500 pCwould be viewed with some concern, whilst a reading of 5000 pC would beconsidered potentially dangerous.

Power authorities typically test transformers by sampling the mineraloil within the transformer about once a year to determine the oil'sdissolved gas concentration by analysis (DGA) and dielectric loss angle(DLA). If high gas readings are obtained, the frequency of sampling isincreased to monthly and even weekly. However, there is always somedelay between the sampling and the analysis in the laboratory. Rapiddeterioration of insulation may not be detected and transformers havefailed catastrophically even when DGA sampling has been carried out.Since it is known that partial discharges of a higher magnitude and/orrepetition rate develop shortly before a major failure, continuousmonitoring of electrical equipment, while it is kept on-line, to provideearly warning, is very desirable.

Partial discharge can be measured using instruments such as Robinson,Haefly or Tettex partial discharge detectors, which detect highfrequency electrical (RF) signals only, by coupling to the lower part ofthe bushing on the transformer or to the windings using capacitordividers and a toroid system. These instruments are normally used in atest bay during high voltage proving tests for a new or re-woundtransformer. These measurements can, however, normally not be undertakenin a substation location due to the high level of electricalinterference. Making reliable readings with these instruments alsorequires considerable skill.

One device for detecting the occurrence of a single partial dischargeevent in a transformer is described in International Application NoPCT/AU94/00263 (WO 94/28566). This device comprised an ultrasonictransducer and a radio frequency antenna that were mounted in thetransformer wall and adapted respectively to detect the ultrasonic andradio frequency pulses generated by a partial discharge. If a radiofrequency signal was detected within a pre-set time period beforedetection of an ultrasonic signal, a partial discharge was assumed tohave occurred. While able to detect such signals, one problem with thedevice described in WO 94/28566 was that electrical noise within thetransformer would generate randomly occurring radio signals that lead tothe triggering of false alarms on occurrences of partial discharge.Shutting down a transformer based on a false alarm is clearlyundesirable and costly.

DISCLOSURE OF THE INVENTION

According to a first aspect, the present invention is an apparatus fordetecting partial discharge in on-line high voltage electrical equipmentcontaining a dielectric, each partial discharge generating a radiofrequency pulse or wave and an ultrasonic pulse or wave, the apparatuscomprising:

at least one transducer means for detecting the ultrasonic pulse or wavegenerated by the occurrence of a partial discharge and subsequentlyoutputting a signal corresponding to this detection:

at least one transducer means for detecting the radio frequency pulse orwave generated by the occurrence of a partial discharge and other radiofrequency pulses or waves generated within the equipment andsubsequently outputting a signal corresponding to this detection; and

a signal processing and analysing means which receives the signalscorresponding to the detection of the radio frequency pulse or wave andultrasonic pulse or wave and which, on receiving a signal correspondingto detection of an ultrasonic pulse or wave, is adapted to:

(a) determine, within a pre-set time period preceding the instance oftime of detection of the ultrasonic pulse or wave, the time delaybetween the instance of detection of all detected radio frequency pulsesor waves and the instance of time of detection of said ultrasonic pulseor wave and generate a spread of time delay values over the pre-set timeperiod:

(b) superimpose the spread of time delay values against other spreads oftime delay values of a plurality of other detected ultrasonic pulses orwaves: and

(c) analyse the superimposed spreads of time delay values to determineif a proportion of the detected ultrasonic pulses or waves are beingdetected at one or more particular time delay values after the time ofdetection of the detected radio frequency pulses or waves.

According to a second aspect, the present invention is a process fordetecting partial discharge in on-line electrical equipment containing adielectric, each partial discharge generating a radio frequency pulse orwave and an ultrasonic pulse or wave, the process comprising the steps:

(i) detecting radio frequency pulses or waves generated within theequipment:

(ii) detecting ultrasonic pulses or waves generated within theequipment:

(iii) on detection of an ultrasonic pulse or wave, determining within apre-set time period preceding the instance of time of detection of theultrasonic pulse or wave, the time delay between the instance ofdetection of all detected radio frequency pulses or waves and theinstance of detection of said ultrasonic pulse or wave and generate aspread of time delay values over the pre-set time period:

(iv) superimposing the spread of time delay values against other spreadsof time delay values of a plurality of other detected ultrasonic pulsesor waves; and

(v) analysing the superimposed spreads of time delay values to determineif a proportion of the detected ultrasonic pulses or waves are beingdetected at one or more particular time delay values after the time ofdetection of the detected radio frequency pulses or waves.

In the above aspects, the superimposing of the spreads of time delayvalues of a plurality of detected ultrasonic pulses leads to theconstructive addition of any identical time delay values from each ofthe spreads in a histogram of counts of time delay values over thepre-set time period. This constructive addition at one or moreparticular time delay values readily distinguishes this particular timedelay value from those determined time delay values that are simply aresult of asynchronous electrical noise within and around the electricalequipment.

The electrical equipment monitored by the apparatus and process in theabove aspects preferably comprises power, instrument, current and highfrequency transformers containing a dielectric, the dielectric being amineral or silicone oil, epoxy or gas. Hereinafter, for the purposes ofclarity, operation of the present invention will be described inrelation to its application to monitoring partial discharge in highvoltage transformers. By on-line, it is to be understood that thepresent invention can monitor the transformers when the transformers arebeing operated or in use. This might include when the transformer isbeing operated under normal operating conditions, but can also includesituations where the transformer is being operated in abnormalconditions or being operated for the purpose of its testing, forexample, during approval tests when overvoltages or impulses areapplied, or the testing of a system that the transformer is a part.

In a preferred embodiment of each aspect, any radio frequency pulsesgenerated within the transformer can be continuously monitored.Similarly, any ultrasonic pulses generated within the transformer can becontinuously monitored. As such, it will be understood that in normaloperation the monitoring of ultrasonic and radio frequency pulses willbe done concurrently. Furthermore, in a preferred embodiment themonitoring of ultrasonic and radio frequency pulses will continue evenwhen the analysing means is analysing received signals.

In a preferred embodiment, the pre-set time period preceding the time ofdetection of the ultrasonic wave is set to be greater than the maximumpossible time delay that could exist between a detected radio frequencypulse and a detected ultrasonic pulse. In one embodiment, the timeperiod can be set at the time of installation of the apparatus in thetransformer. In another embodiment, the time period can be adjustablefollowing installation of the apparatus. In a typical transformer, thepre-set time period might be set between 1 ms and 10 ms, more preferablybetween 2 ms and 6 ms and still more preferably at or about 4 ms. Thispre-set time period is determined by the data storage capabilities ofthe apparatus and should take into account the largest internaldimension of the transformer (normally the diagonal distance from cornerto corner), and the speeds of ultrasonic pulses in the materials thatcomprise the internal parts of the transformer such as the dielectric,laminated iron core, copper windings and the like.

In the case where the pre-set time prior to the ultrasonic pulse is setat 4 milliseconds and the apparatus is sampling in 60 microsecondblocks, it will be appreciated that there are a maximum of 66 sampletime delay periods that can constitute the spread of time delay valuescombined in the histogram.

In superimposing a plurality of spreads of time delay values, the signalprocessing software and analysing means can superimpose all spreadsgenerated within a particular time period. The time period can rangefrom a few milliseconds to minutes and even hours, if desired. In oneembodiment, the analysing means can superimpose all spreads generated ina particular time period just preceding the step of superimposing thespreads. The time period can be between 0.1 and 10 seconds, morepreferably 1 and 8 seconds, and even more preferably be about 2 seconds.The analysing means can be adapted to continuously update thesuperimposition of the spreads of generated time delay values, sodiscarding, those spreads generated earlier than the particular timeperiod. For example, the superimposition at any particular time willonly include those spreads of time delay values generated in theparticular time period preceding this time. In one embodiment, theparticular time period can be pre-set at the time of installation of theapparatus in a transformer. In another embodiment, the time period canbe adjustable following installation of the apparatus.

In a preferred embodiment, the analysing means comprises amicroprocessor means under the control of appropriate softwareinstructions. The microprocessor means can be physically located closeto the transformer or can be located at a distant location.

The software instructions of the microprocessor can be adapted tostatistically count the number of determined time delay values within aspread of time delay values and superimpose these counts with the countsmade from a plurality of such spreads generated on detection of otherultrasonic pulses within the particular pre-set time period to form ahistogram of counts versus time delay value. Since the time delay valuefor a particular partial discharge location does not vary, a peakdevelops in the histogram corresponding to the time delay value betweenthe radio frequency pulse and ultrasonic pulse being generated by thepartial discharge. If there are two sites of partial discharge withinthe transformer, two peaks would be generated in the histogram so longas the distance between each site and the transducer was different. As,in operation, the analysing means is required to superimpose a pluralityof spreads of time delay values, it will be appreciated that theanalysing means has a means of processing such a plurality of spreads atleast for a time sufficient to allow formation of the histogram ofcounts versus time delay value. However, if the time delay is the samefor one head it will be different for another head. Typically, two,three or four heads can be installed depending on the type, size, designand power rating of the transformer to address this issue.

In a preferred embodiment, the ultrasonic transducer means of detectionof an ultrasonic pulse also outputs a signal representative of theamplitude of the detected ultrasonic pulse. In this embodiment, theanalysing means is preferably adapted to receive this amplitude signal.The analysing means can be adapted to disregard spurious signalsrepresentative of ultrasonic pulses less than a pre-determined amplitudesetting. In one embodiment, the analysing means can be adapted toundertake no further analysis of the ultrasonic signal if the amplitudeof the ultrasonic pulse that lead to the generation of that signal isbelow a pre-determined amplitude setting. The pre-determined amplitudesetting can be pre-set at the time of installation of the apparatus inthe transformer. In another embodiment, the pre-determined amplitudesetting can be adjustable following installation of the apparatus. Itwill also be appreciated that some ultrasonic pulses may be generatedthat are below the detection threshold of the ultrasonic transducer.

In another embodiment, the radio frequency transducer means of detectionof a radio frequency pulse also outputs a signal representative of theamplitude of the detected radio frequency pulse. In this embodiment, theanalysing means is preferably adapted to receive this amplitude signal.The analysing means can be adapted to disregard signals representativeof radio frequency pulses less than a pre-determined amplitude setting.The predetermined amplitude setting for radio frequency pulses can bepre-set at the time of installation of the apparatus in the transformer.In another embodiment, the pre-determined amplitude setting can beadjustable following installation of the apparatus. It will also beappreciated that some radio frequency pulses may be generated in thetransformer that are below the detection threshold of the radiofrequency transducer.

In one embodiment, the analysing means can be adapted to activate analarm means if the results of an analysis of the received signals over aperiod meets criteria considered indicative of partial discharge. Thecriteria considered indicative of partial discharge can be pre-set atthe time of installation of the apparatus in the transformer. In anotherembodiment, the criteria can be adjusted following installation.

In one embodiment, the analysing means can be adapted to activatevarying types of alarm means depending on the nature of the detectedultrasonic pulse. For example, the analysing means, on receiving anultrasonic signal and determining that there is a peak in counts at oneor more particular time delay values between detection of the radiofrequency and ultrasonic pulses, can determine if the ultrasonic pulsehas a magnitude greater than the pre-determined amplitude setting. Ifthe amplitude is greater than the pre-determined amplitude settingand/or there is considerable repetition of generation of ultrasonicpulses above some pre-determined setting, the signal processing softwareanalysing means can activate a specific type of alarm means. Forexample, this set of conditions may be said to activate a Class 1 alarm.If a peak at a time delay value has been determined but the amplitudeand/or repetition of generation of the ultrasonic pulses is below thepre-determined setting, then this set of conditions may be said toactivate a Class 2 alarm.

If the analysing means receives signals representative of ultrasonicpulses that are greater than the pre-determined amplitude and/orrepetition setting but does not determine that there is any peak at anytime delay value between the signals and any preceding radio frequencysignals, this set of conditions may be said to activate a Class 3 alarm.Finally, if ultrasonic pulses are detected that are less than thepre-determined amplitude and/or repetition setting but again without anydetermination of a peak at any time delay value, this set of conditionsmay be said to activate a Class 4 alarm. In still a further embodiment,the pre-determined amplitude and/or repetition setting in the analysingmeans can be different in the case where there has been no determinationof a peak at any time delay value to that when there has been adetermination of a peak at some time delay value. For example, if a peakin counts at a time delay value is determined, the pre-determinedsetting might be set lower than the setting in the case where no peak ata time delay value between a detected ultrasonic pulse and radiofrequency pulse has been determined.

By activating different classes of alarms, the analysing means providesan organisation monitoring the performance of a transformer (forexample, a power generation or distribution authority) the ability todetermine the severity of the fault in the transformer. For example,activation of a Class 4 alarm might not be considered a significantcause of concern but may warrant that this transformer should be moreclosely monitored or undergo routine testing and maintenance at anearlier date than had otherwise been planned. In contrast, activation ofa Class 1 alarm might be considered by the monitoring organisation asworthy of immediate or relatively quick shutdown of the transformer toallow on-site testing and, if necessary, repair of the fault eitheron-site or in a repair shop. If over a period of minutes, hours, days oreven weeks, the apparatus activates firstly a Class 4 alarm, followed bya Class 3, or Class 2 and then a Class 1 alarm, this provides themonitoring organisation with an indication of the rate of increase ofseverity of the fault in the transformer and gives an indication of whenthe transformer should undergo inspection. Rapid change from activationof a Class 4 alarm to a Class 3 alarm or a Class 2 alarm to a Class 1alarm would be considered very dangerous, for example, and normally leadto the transformer being taken off-line and shutdown.

The alarm means can comprise both visual and/or audible means. Thevisual alarm means can comprise bright flashing coloured lights, lightemitting diodes (LEDs), or similar devices and may be integrated intopre-existing software monitoring systems such as SCADA. Harley, Citect,etc. It will be appreciated that an alarm condition might be indicatedby the turning on of a light or the extinguishment of a light. In someinstances, the latter is preferred as any failure of the light isreadily noted and can be corrected. In an alternative embodiment, thealarm means might comprise an appropriate message on a television screenor computer monitor display. The audible alarm means might comprise abell, buzzer, siren or other similar device. It has been found that acontinuous 3 kHz sound from a tweeter is particularly effective. Thealarm means can be physically located close to the monitored transformeror at some distant location.

In another embodiment, the analysing means can include a data storagemeans adapted to store all signals received from the respectivetransducer means, and/or the generated superimpositions of spreads oftime delay values. The data storage means might be adapted to only storemost recently logged data within a pre-set time period or only storedata that is indicative of instances of partial discharge. For example,the storage means might only store all logged data from a preceding 24hour period.

The stored data can be downloaded to a controlling means at a local orremote location either on request of an operator or automatically. Inone embodiment, the controlling means or the analysing means mightroutinely initiate a data transfer from the analysing means on aparticular day or at a particular time. For example, where a storagemeans is adapted to store data for 24 hours, the controlling means oranalysing means might initiate a data transfer from the analysing meansalso about every 24 hours. Other time periods for data transfer can bereadily envisaged.

If the controlling means notes that the microprocessor has initiated analarm means, it can provide an appropriate indication to personnelresponsible for monitoring the performance of the transformer. Such anappropriate indication might comprise a visual and/or audible indicationon a computer monitor. The controlling means can also includeappropriate data storage means to allow storage of all data transferredfrom the analysing means. The system allows archiving of the alarms andthe data over periods of many months or even years. This data storagewould allow responsible personnel in the monitoring organisation tocompare monitored transformer performance against that monitored at someearlier time and so determine if there has been change in theperformance, ie trend analysis. In a further embodiment, the controllingmeans would preferably be adapted to receive data transferred from aplurality of analysing means monitoring a number of differenttransformers. For example, the controlling means belonging to a powergeneration or distribution authority could be networked to all or someproportion of its transformers in the network that have the apparatusaccording to the present invention installed. This allows the powerdistribution authority to monitor the performance of the transformers inits network without the necessity to physically have maintenancepersonnel attend at the location of each transformer.

In one embodiment, the respective transducer means can be housed withina common enclosure. The enclosure can be positioned within the wall ofthe transformer such that one surface of each transducer means iscoincident with the interior surface of the wall.

The transducer means for detection of ultrasonic pulses preferablycomprises a piezoelectric element. The piezoelectric element preferablyhas a first face and a second face. The piezoelectric element preferablyhas a thickness resonant frequency between about 50 and about 300 kHz,more preferably 60 to 250 kHz, and most preferably at about 190 kHz. Thepiezoelectric element preferably has a maximum operating temperature ofat least 100°C. and more preferably at least 120° C. The piezoelectricelement preferably can also withstand mechanical vibrations at least upto 5 g.

The piezoelectric element is further preferably a ceramic/polymercomposite. The piezoelectric element preferably has a 1-3 geometry. Theceramic can be selected from the group comprising poly-crystalline leadtitanate, lead zirconate titanate (PZT), lead niobate or bariumtitanate. The polymer is preferably a thermosetting polymer. Thethermosetting polymer can be selected from the group comprising epoxyresin, polyurethane, silicone or Bakelite.

The ceramic in the piezoelectric element can be fabricated by sinteringand firing oxides or carbonates of barium, titanate, zirconate and/orlead to form a ceramic disc. The opposing parallel faces of the ceramicdisc are then preferably coated with a suitable conductive material toform electrodes. The ceramic disc is then preferably poled by immersingthe disc in hot oil and applying a DC electric field to the disc whilstit is held at a temperature of about 90° C. The oil is then preferablyallowed to cool to room temperature whilst the electric field ismaintained across the disc.

Once poled, the disc is mounted on an aluminium or epoxy block byadhering one face of the disc to the block using an epoxy adhesive. Theblock is then preferably carefully gripped in the chuck of a diamond sawcutting machine to avoid damage to the ceramic disc. The ceramic disc isthen preferably sliced with a diamond edged saw to form a series ofspaced parallel cuts. The disc is then preferably cleaned with methanolto remove any debris and then vacuum back filled with the thermosettingpolymer, such as epoxy. Any excess epoxy is preferably removed bylapping before the disc is again preferably sliced by the cuttingmachine to form a further series of spaced parallel cuts that are atright angles to the first set of cuts. The disc is then cleaned withmethanol before an outer casing of slightly greater diameter and heightis preferably positioned around the disc before the disc and surroundingcasing is vacuum back-filled with epoxy. The result is a set of parallelceramic columns or pegs supported in the thermoset epoxy.

The outer casing provides extra support for the outer pegs in thecomposite and helps to prevent any inadvertent breakage of them,especially when the composite is subsequently sliced from the supportingblock. The outer casing is also preferably formed from a thermosettingpolymer, such as epoxy. The outer casing serves to further decrease thelateral sensitivity of the transducer to shear waves and lateral highfrequency vibrations in the transformer wall that are unrelated toultrasonic waves due to partial discharge. Once removed from thesupporting block, the first and second surfaces of the composite arepreferably lapped using 120, 400 and 600 grade emery paper,respectively, to expose the ceramic columns.

In a preferred embodiment, the first and second surfaces are coated withan electrically conductive adhesive, such as silver loaded epoxy.Following application and before setting of the silver loaded epoxy,electrically conductive gauze electrodes are pressed into and adheredwith the silver loaded epoxy to form electrodes for the compositetransducer. The electrically conductive gauze is preferably metallicgauze, and even more preferably can be brass gauze. On adhesion, thediameter of the metallic gauze is preferably greater than that of thefirst and second surfaces of the composite. Once applied, the gauze canbe trimmed to the diameter of the first and second surfaces,respectively. In each case, a small tag of gauze is preferably retainedto allow ready strong, reliable, ohmic electrical connection to theelectrodes. Electrical connection to the tags is preferably provided byinsulated copper wire that is soldered to each tag.

The composite transducer preferably has a short ring down time so thatit recovers quickly from detection of an ultrasonic pulse and is readyto detect the next one. To increase the damping of the transducer, abacking plate can be cemented to the second surface of the transducer.The backing plate is preferably formed from a tungsten loaded epoxy.

A matching layer can also be attached to the first surface of thecomposite transducer. The matching layer can comprise one or more layersof the thermosetting polymer used in the composite. The thickness ofthis matching layer is preferably a quarter wavelength of the transducerthickness resonant frequency. The matching layer acts as an acousticimpedance converter between the higher acoustic impedance of the pegsand that of the oil thus improving the acoustic impedance matching ofthe composite overall. The acoustic impedance of the transducer ispreferably as close as possible to the acoustic impedance of the oil soas to minimise the reflections of longitudinal ultrasonic waves at thefirst surface of the transducer. For maximum transfer, the matchinglayer is preferably the geometric mean of the composite and the oil. Thematching layer also acts as a wear plate to protect the composite duringuse.

The composite transducer can have a tuning inductor electricallyconnected between the copper wires connected to the first and secondsurface electrodes to further enhance the sensitivity. The tuninginductor is preferably shielded to prevent magnetic pickup by theinductor in the transformer environment.

Once manufactured, each composite transducer is preferably tested usingan impedance analyser to measure the electromechanical coupling of thetransducer, the electromechanical coupling being a measure of theefficiency of the transducer in converting mechanical energy due to theultrasonic waves into electrical energy.

In another embodiment for gas filled transformers, the ultrasonictransducer can be manufactured from a piezoelectric polymeric material.In one preferred embodiment, the piezoelectric polymeric material can bepolyvinylidene fluoride (PVDF).

The transducer means for detection of radio frequency pulses cancomprise an antenna selected from the group of a ferrite core aerial, atuned circuit, or a capacitive metal plate. The capacitive metal plateis preferred and preferably has a capacitance to ground of between 20 pFand 250 pF. The plate preferably has an annular geometry and can befabricated from brass. The area of the annular plate, the dielectricconstant of the insulant in the transformer and the spacing of theannular plate and supporting adjustable spacers from the transformerwall (which is earthed and forms the other plate of the capacitor)determines the capacitance value.

Within the common enclosure, the piezoelectric element is preferablypositioned coaxially within the capacitive annular plate and adaptedsuch that the dielectric surrounds the capacitive plate and all but oneface of the outer casing of the piezoelectric transducer. It ispreferred that all air bubbles in the dielectric be removed by degassingprior to and after installation of the transducers. A bleed hole can beprovided to ensure that the space about the transducers is completelyfilled with mineral oil.

Both the ultrasonic and radio frequency transducers can be electricallyand mechanically attached to a lead through plate using thick copperwires and one or more bolts. Adjustable spacers around the boltssupporting the annular brass plate can allow the distance between theplate and the transformer wall to be set at a desired spacing therebyallowing for adjustment of the capacitance of the radio frequencytransducer.

The lead through plate preferably consists of an epoxy moulded platewith brass insert threaded connectors. If a copper wire is attached atone side of the plate and a copper wire is attached at the other side ofthe epoxy plate, electrical continuity is provided through the epoxyplate from one side to the other. Typically, the lead through plate hasseveral brass insert connectors. One of these brass insert connectorscan be used to support the ultrasonic transducer using a bolt which iscast into the transducer, two insert connectors can be used to connectthe thick copper wires from the ultrasonic transducer and three or moreinsert connectors can be used to support the brass annular plate beingused as the radio frequency antenna.

The epoxy lead through plate, with the transducers attached, is locatedthrough a hole in the transformer wall or through an inspection cover sothe transducers are inside the transformer. The lead through plate isthen preferably sealed with neoprene O rings, or rubberised cork gasketsand held in position with metal flanges. Outside the transformer a metalenclosure is attached to the flanges.

Electrical connections using shielded coaxial cables are preferably madevia the brass insert connectors in the epoxy lead through plate from thetransducers to ultrasonic transducer circuitry and radio frequencytransducer circuitry. This circuitry is preferably housed in a smallmetal box. This small box preferably fits into the larger metalenclosure attached to the flanges. Shielded twisted pair cables are thenpreferably connected from the outputs of the ultrasonic transducercircuitry and the radio frequency transducer circuitry in the metal boxto insulated lead through connectors in the side of the metal enclosure.The lid of the enclosure, which preferably sits on a rubberised corkseal, is then preferably clamped in position so that the wholearrangement is waterproof and termite and vermin resistant.

The ultrasonic transducer processor that transmits the signals betweenthe transducer and the analysing means preferably comprises in sequencean amplifier, a precision rectifier, a 125 kHz high pass filter, a 1 kHzlow pass filter, an amplifier and a buffer with an analogue output. Theradio frequency transducer processor that transmits the signals betweenthe radio frequency transducer and the analysing means preferablycomprises in sequence a 1 to 70 MHz pre-amplifier, a precisionrectifier, a 1 MHz low pass filter, a high speed comparator and amonoshot with optically isolated digital output. All of the electroniccomponents in the electronic processors are preferably capable ofwithstanding high temperature ie, military specification 120° C.,because of the possible temperatures in the head enclosure on top of thetransformer.

Where necessary, hereinafter, the transducers, epoxy lead through plate,connectors, shielded cables, seals, spacers, metal enclosure, ultrasonictransducer circuitry, radio frequency circuitry and lid are referred tocollectively as the “head”.

From the outputs of the ultrasonic and radio frequency transducercircuitry, electrical output signals are provided to the microprocessorof the analysing means via twisted pair shielded cable. The output ofthe microprocessor can be coupled by optical fibre cable to a computerhaving a coupled modem. The computer can be adapted to display alarmconditions on a computer monitor or can transmit the alarm conditionsvia the modem and a telephone or other communications network to theremotely located controlling means.

Normally, for most installations the system has to be able to withstandenvironmental temperatures in the range −25° C. to +120° C. However,with special components this range can be extended to −35°C. to +120° C.A special electronic cut-out device set at +110° C. can be incorporatedto shutdown and protect the system. The possible working range istherefore −35°C. to +110° C., which is adequate for most installations.

In a further embodiment, the location in 3D of a partial dischargesource within a high voltage transformer can be determined by analysisof the signal outputs from at least three transducer heads housed withinthe wall of the transformer. If a partial discharge is detected, theanalyser means preferably determines the time delay values at eachtransducer head and then estimates the location of the partial dischargesource by triangulation. It will be understood that the resolution oflocation can be improved by allowing for variation in the velocities ofultrasonic waves in the different materials in the transformer, allowingfor refraction at interfaces and by iteration finding the most probableacoustic paths for the ultrasonic pulse from the partial dischargesource to each of the three heads.

In a still further embodiment, following installation of an apparatusaccording to the present invention in a transformer, the apparatus canbe tested and calibrated. Such testing and calibration can be undertakenby positioning a partial discharge generator in the transformer tosimulate a fault in the transformer's insulation. By being able to varythe level of partial discharge injected into the transformer, it ispossible to set the sensitivity levels of the apparatus. Once testingand calibration is complete, the partial discharge generator can beremoved.

According to a further aspect, the present invention is a transducerunit comprising:

a piezoelectric composite disc for detecting ultrasonic pulses or wavesand subsequently outputting a signal corresponding to this detection,the piezoelectric element having electrodes formed on a first surfaceand a second surface, the electrodes being formed of an electricallyconductive metal gauze material pressed into an electrically conductiveadhesive; and

an annular capacitive plate for detecting radio frequency pulses orwaves and subsequently outputting an electrical signal corresponding tothis detection.

In a preferred embodiment of this further aspect, the electricallyconductive adhesive is a silver loaded epoxy coated on the first andsecond surfaces. The electrically conductive gauze is preferably a brassor copper gauze. The diameter of the metallic gauze is preferablygreater than that of the first and second surfaces of the composite.Once applied, the gauze can be trimmed to the diameter of the first andsecond surfaces, respectively. In each case, a small tag of gauze ispreferably retained to allow ready electrical connection to theelectrodes. Electrical connection to the tags is preferably provided byinsulated or enamelled copper wire that is soldered to each tag.

In other embodiments, the transducer unit can have the features asdescribed above with respect to the first aspect of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example only, a preferred embodiment of the present inventionwill be described with reference to the accompanying drawings in which;

FIG. 1 is a partly cross-sectional view illustrating a transducer headof an apparatus according to the present invention positioned in thewall of a transformer;

FIG. 2 is a cross-sectional view of the ultrasonic pulse detector withinthe transducer head;

FIG. 3 is a schematic view of the apparatus according to the presentinvention;

FIG. 4 is a schematic view of the ultrasonic signal and radio frequencysignal processors in the apparatus depicted in FIG. 3;

FIG. 5 is a graph depicting processor output versus time of oneembodiment of microprocessor means in the apparatus according to thepresent invention following detection of a radio frequency signal and anultrasonic signal;

FIG. 6 is a histogram of counts from a plurality of spreads of timedelay values generated by the microprocessor means of the presentinvention; and

FIG. 7 is a flowchart for the generation of different alarm classes bythe microprocessor means according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The transducer head of an apparatus for the monitoring of partialdischarge in on-line high voltage transformers is generally shown as 10in the drawings.

As depicted in FIG. 1, the transducer head 10 is mounted within a wall11 of a transformer 12 containing mineral oil 13. The transducer head 10comprises a metal enclosure 14 having a metal lid 14 a, inside which isan annular capacitive brass plate 15, for the detection of radiofrequency pulses, and an ultrasonic transducer 16 acting as a detectorof ultrasonic pulses. The plate 15 and transducer 16 are positioned soas to be coincident with the interior surface of the wall 11 of thetransformer 12, although this is not critical.

As is depicted more clearly in FIG. 2, the ultrasonic transducer 16 is aceramic/polymer composite having a 1-3 geometry, ie. the ceramic isaligned in 1 direction whilst the polymer has 3 directions. The ceramicwithin the transducer 16 is lead zirconate titanate (PZT), however,other suitable piezoelectric ceramic materials could be utilised. Thepolymer in the composite is a thermosetting epoxy resin. Duringfabrication, the ultrasonic transducer 16 is encapsulated within anepoxy casing 29. Once installed in the transducer head 10, the casing 29is mounted by a mounting bolt 31 to a lead through plate 32. The casing29 has a first face 33 and a second face 34.

Within the casing 29 and extending outwardly from the first face 33 isan epoxy matching layer 35. The matching layer 35 is impervious to themineral oil 13 and has low acoustic impedance in mineral oil forultrasonic pulses, thereby allowing detection with little reflection ofan ultrasonic pulse from a partial discharge. The thickness of thematching layer 35 is a quarter wavelength at the transducer thicknessresonant frequency. Extending outwardly from the second face 34 is abacking plate 36 that is formed from tungsten particle loaded epoxyresin that is substantially opaque to ultrasonic pulses. The tungstenloading of the epoxy in the backing plate 36 serves to ensure noreflection of ultrasonic signals is detected by the ultrasonictransducer 16 from the lead through plate 32 and also helps preventultrasonic pulses, propagating in the wall 11 of the transformer 12,being transmitted to the ultrasonic transducer 16.

Each face of the ultrasonic transducer 16 has an electrode 37 for theaccumulation of charge. Each electrode 37 comprises a layer of silverloaded epoxy adhesive into which has been pressed a thin brass gauzesheet.

Electrical connection from each of the electrodes 37 is by way of tinnedcopper wire 38 and enamelled copper wire 39. While not depicted, atuning indicator can be electrically connected between the copper wires.

The ultrasonic transducer 16 is positioned coaxially within the annularplate 15 such that the mineral oil 13 surrounds the plate 15 and all butone face of the casing 29. The annular plate 15 can have a capacitanceto ground of between 20 and 250 pF and is made of brass.

The lead through plate 32, which is held in place by flanges 9, consistsof an epoxy moulded plate with brass insert threaded connectors. Twobrass insert connectors in the lead through plate 32 can be used as theelectrical connection through the plate 32 from the copper wire 39extending from the ultrasonic transducer 16 to coaxial cables extendingto the ultrasonic transducer circuitry 40. Two or three adjustable brassinsert threaded connectors 15 a are used to support the annular plate 15and also provide electrical connection for the electrical signalsreceived by the plate 15 to travel through the plate 32 to the radiofrequency signal processor 21 (see FIG. 3).

The ultrasonic electronic circuit signal processor 40 and the radiofrequency signal processor 21, are housed in a small metal box 17 withinthe enclosure 14 mounted to the external wall of the transformer 12. Anelectromagnetic noise shield 18 also surrounds the plate 32. Power forthe processors 21, 40 is provided by a power supply (not depicted) thatsupplies power through cable 19 that enters the enclosure 14.

As depicted in FIG. 4., the ultrasonic detector signal processor 40 thatreceives the signals from the ultrasonic transducer 16 and passes themto the analysing means 50 comprises in sequence a pre-amplifier 41, aprecision rectifier 42, a 125 kHz high pass filter 43, a 1 kHz low passfilter 44, an amplifier and a buffer 45 having an analogue output 46.

The radio frequency signal processor 21 comprises in sequence a 1 to 70MHz pre-amplifier 22, a precision rectifier 23, a 1 MHz low pass filter24, a high speed comparator 25 and a monoshot 26 with digital output 27.

The amplified electrical signals from the radio frequency output 27 andultrasonic output 46 of the transducer head 10 are transmitted viacoaxial shielded cables to the analysing means 50.

The analysing means 50 comprises a microprocessor means 51 thatprocesses the incoming signals under the control of appropriate softwareinstructions.

Either at installation or following installation, the software of themicroprocessor means 51 is calibrated. This calibration includes settinga value that will be the pre-set time period preceding the time ofdetection of an ultrasonic pulse. The pre-set time period would normallybe set to be greater than the maximum possible time delay that couldexist between a detected radio frequency pulse and a detected ultrasonicpulse given the dimensions of the transformer 12 in which the apparatusis being installed. In a normal case, the pre-set time period in thedepicted microprocessor means 51 would be set at about 4 ms.

At the time of calibration, it would also be normal to set theparticular time period from which the microprocessor means 51 willselect and superimpose all spreads of time delay values measured betweendetected radio frequency and ultrasonic pulses. In a normal case, andfor the purpose of the following description, the particular time periodis set as the preceding 2 seconds.

At the time of calibration, it would also be normal to set thesensitivity of the apparatus. For example, the microprocessor means 51may be set up to ignore signals transmitted from the ultrasonictransducer 16 and/or the radio frequency detector 15 that are below aparticular amplitude and/or rate of repetition. During calibration, thepre-determined amplitude setting that establishes the class of alarmraised by the microprocessor means 51 can also be pre-set.

In operation, the microprocessor means 51 on receiving a signal fromcircuitry 40 corresponding to an ultrasonic pulse, determines, withinthe preset time period of 4 ms, the time delay between the detection ofany detected radio frequency pulses and the detection of the ultrasonicpulse and generates a spread of counts of time delay values over the 4ms period. This spread of time delay values is then superimposed by themicroprocessor means 51 with other determined spreads of time delayvalues generated in the preceding 2 seconds. The superimposition of theplurality of spreads is used to form a histogram as depicted as FIG. 6.Since the time delay value, as illustrated in FIG. 5, for a particularpartial discharge location does not vary a peak develops in thehistogram which distinguishes this time delay value from those othermeasured time delay values that were a product of asynchronouselectrical noise within the transformer. While not depicted, if therewere two sites of partial discharge within the transformer, it would beenvisaged that two peaks would be generated in the histogram so long asthe distance between each site and the transducer head was different.

Once the microprocessor means 51 has determined that a time delaybetween detected radio frequency and ultrasonic pulses is present in thetransformer, the characteristics of the detected ultrasonic pulse can becompared against pre-determined settings. As depicted in FIG. 7, if theamplitude and/or repetition of the ultrasonic pulses are greater than apre-determined setting considered to represent a high ultrasonic level,the microprocessor means can activate a Class 1 alarm type. If theultrasonic amplitude and/or repetition is not of a high level, themicroprocessor means 51 can activate a Class 2 alarm type.

As is also depicted in FIG. 6, the microprocessor means 51 can beadapted to still activate an alarm even in those instances where no peaktime delay value has been determined. For example, different alarm typescan be activated simply on detection of high or medium levels ofultrasonic pulses.

By being able to activate different classes of alarm, the microprocessormeans 51 can be used as an indicator, not just of the presence of afault in the transformer's insulation, but also as an indicator of theseverity of the fault and the rate of degradation of the transformer.For example, activation of a Class 4 alarm type might not be considereda significant cause of concern but may at least warrant that thistransformer be monitored more closely in future. It may also mean thatthe transformer should undergo routine maintenance sooner than had beenanticipated. In contrast, activation of a Class 1 alarm type might beconsidered a significant cause of concern and justify immediate orrelatively quick shutdown of the transformer to allow appropriateon-site testing.

It would be anticipated that at least a Class 2 alarm type and possiblya Class 3 or 4 alarm type would be activated before the activation of aClass 1 alarm type in the case where a fault was gradually occurring inthe insulation of a transformer. As such, by monitoring the rate ofactivation of the alarm types from Class 4 to Class 1 also provides anindication of how quickly the severity of the fault is increasing in thetransformer's insulation.

In the depicted embodiment, the alarm that is activated by themicroprocessor means 51 comprises a message on a computer monitor 52that indicates the class of alarm activated by the microprocessor means51. A corresponding audible alarm can also be activated.

The analysing means 50 can be networked to a central controller 60, suchas a PC monitor display and data logger. The PC monitor display and datalogger can be located close to or very distant from the transformer 12being monitored. For example, the analysing means 50 and PC monitordisplay and data logger 60 can be networked by a telecommunicationsnetwork, such as a telephone network. In one system, it can be envisagedthat analysing means 50 is in continuous communication with the PCmonitor display and data logger 60. In another embodiment, a modemconnected to the analysing means 50 can be adapted to dial a modem inthe PC monitor display and data logger 60 or vice versa and allowtransfer of data as appropriate. The dialing of the modem could be doneperiodically as was considered necessary by the monitoring organisation.While the analysing means 50 would download data to the PC monitordisplay and data logger 60, it will be appreciated that the PC monitordisplay and data logger 60 could provide instructions to the analysingmeans 50. Such instructions could be used to allow on-line adjustmentsto the settings of the software instructions running on themicroprocessor means 51.

While the PC monitor display and data logger 60 is depicted networked tojust one analysing means 50, it will be appreciated that it could benetworked to a plurality of analysing means 50 mounted to differenttransformers in many different locations. This allows, for example, apower distribution authority to monitor the performance of thetransformers in its network without the necessity of having maintenancepersonnel attend at the location of each transformer. This is attractivefor distant remote locations in country areas.

It will be appreciated by persons skilled in the art that numerousvariations and/or modifications may be made to the invention as shown inthe specific embodiments without departing from the spirit or scope ofthe invention as broadly described. The present embodiments are,therefore, to be considered in all respects as illustrative and notrestrictive.

What is claimed is:
 1. An apparatus for detecting partial discharge inon-line high voltage electrical equipment containing a dielectric eachpartial discharge generating a radio frequency pulse or wave and anultrasonic pulse or wave, the apparatus comprising: at least onetransducer means for detecting the ultrasonic pulse or wave generated bythe occurrence of a partial discharge and subsequently outputting asignal corresponding to this detection; at least one transducer meansfor detecting the radio frequency pulse or wave generated by theoccurrence of a partial discharge and other radio frequency pulses orwaves generated within or external to the equipment and subsequentlyoutputting a signal corresponding to this detection; and a signalprocessing and analysing means which receives the signals correspondingto the detection of the radio frequency and ultrasonic pulses or wavesand which on receiving a signal corresponding to detection of anultrasonic pulse or wave is adapted to: (a) determine within a pre-settime period preceding the instance of time of detection of theultrasonic pulse or wave the time delay between the instance ofdetection of all detected radio frequency pulses or waves and theinstance of detection of said ultrasonic pulse or wave and generate aspread of time delay values over the pre-set time period; (b)superimpose the spread of time delay values against other spreads oftime delay values of a plurality of other detected ultrasonic pulses orwaves; and (c) analyse the superimposed spreads of time delay values todetermine if a proportion of the detected ultrasonic pulses or waves arebeing detected at one or more particular time delay values after thetime of detection of the detected radio frequency pulses or waves. 2.The apparatus of claim 1 wherein the respective transducer meanscontinuously monitor the electrical equipment.
 3. The apparatus of claim1 wherein the pre-set time period preceding the time of detection of theultrasonic pulse is set to be greater than the maximum possible timedelay that could exist between a detected radio frequency pulse and adetected ultrasonic pulse in the electrical equipment.
 4. The apparatusof claim 1 wherein the pre-set time period is set between 1 ms and 10ms, more preferably between 2 ms and 6 ms and still more preferably ator about 4 ms.
 5. The apparatus of claim 1 wherein the signal processingsoftware and analysing means superimpose all spreads generated within aparticular time period.
 6. The apparatus of claim 5 wherein theanalysing means superimposes all spreads generated in a particular timeperiod just preceding the step of superimposing the spreads.
 7. Theapparatus of claim 6 wherein the time period can be between 0.1 and 10seconds, more preferably 1 and 8 seconds and even more preferably beabout 2 seconds.
 8. The apparatus of claim 1 wherein the analysing meanscomprises a microprocessor means under the control of appropriatesoftware instruction.
 9. The apparatus of claim 8 wherein the softwareof the microprocessor is adapted to statistically count the number ofdetermined time delay values within a spread of time delay values andsuperimpose these counts with the counts made from a plurality of suchspreads generated on detection of other ultrasonic pulses within theparticular pre-set time period to form a histogram of counts versus timedelay value.
 10. The apparatus of claim 1 wherein the ultrasonictransducer means on detection of an ultrasonic pulse also outputs asignal representative of the amplitude of the detected ultrasonic pulse.11. The apparatus of claim 10 wherein the analysing means is adapted toundertake no further analysis of the ultrasonic signal if the amplitudeof the ultrasonic pulse that lead to the generation of that signal isbelow a pre-determined amplitude setting.
 12. The apparatus of claim 1wherein the radio frequency transducer means on detection of a radiofrequency pulse also outputs a signal representative of the amplitude ofthe detected radio frequency pulse.
 13. The apparatus of claim 12wherein the analysing means is adapted to disregard signalsrepresentative of radio frequency pulse less than a pre-determinedamplitude setting.
 14. The apparatus of claim 1 wherein the analysingmeans is adapted to activate varying types of alarm means depending onthe nature of the detected ultrasonic pulse.
 15. The apparatus of claim14 wherein the alarm means comprise visual and/or audible means.
 16. Theapparatus of claim 14 wherein the alarm means is physically locatedclose to the monitored transformer or at some distant location.
 17. Theapparatus of claim 14 wherein the analysing means includes a datastorage means adapted to store all signals received from the respectivetransducer means, and/or the generated superimpositions of spreads oftime delay values.
 18. The apparatus of claim 17 wherein the data storedon the data storage means is downloadable to a controlling means at alocal or remote location either on request of an operator orautomatically.
 19. The apparatus of claim 18 wherein if the controllingmeans receives a signal indicative of activation of an alarm means, itcan provide an appropriate indication to personnel responsible formonitoring the performance of the transformer.
 20. The apparatus ofclaim 18 wherein the controlling means is adapted to receive datatransferred from a plurality of different analysing means.
 21. Theapparatus of claim 1 wherein the respective transducer means are housedwithin a common enclosure that is positionable within a wall of theelectrical equipment such that one surface of each transducer means iscoincident with the interior surface of the wall.
 22. The apparatus ofclaim 1 wherein the transducer means for detection of ultrasonic pulsescomprises a piezoelectric element having a first face and a second face,the piezoelectric element having a thickness resonant frequency betweenabout 50 and about 300 kHz, more preferably 60 to 250 kHz, and mostpreferably at about 190 kHz.
 23. The apparatus of claim 22 wherein thepiezoelectric element is a ceramic/polymer composite having a 1-3geometry, the ceramic being selected from the group comprisingpoly-crystalline lead titanate, lead zirconate titanate (PZT), leadniobate or barium titanate.
 24. The apparatus of claim 1 wherein thetransducer means for detection of radio frequency pulses is an antennaselected from the group of a ferrite core aerial, a tuned circuit, or acapacitive metal plate.
 25. The apparatus of claim 24 wherein theantenna is a capacitive metal plate having an annular geometry.
 26. Theapparatus of claim 25 wherein the piezoelectric element is positionedcoaxially within the capacitive annular plate.
 27. The apparatus ofclaim 1 wherein the location of a partial discharge source within highvoltage electrical equipment is determined by analysis of the signaloutputs from at least three transducer heads housed within a wall of theelectrical equipment.
 28. A process for detecting partial discharge inon-line electrical equipment containing a dielectric, each partialdischarge generating a radio frequency pulse or wave and an ultrasonicpulse or wave, the process comprising the steps: (i) detecting radiofrequency pulses or waves generated within the equipment; (ii) detectingultrasonic pulses or waves generated within the equipment; (iii) ondetection of an ultrasonic pulse or wave, determining within a pre-settime period preceding the instance of time of detection of theultrasonic pulse or wave, the time delay between the instance ofdetection of all detected radio frequency pulses or waves and theinstance of detection of said ultrasonic pulse or wave and generate aspread of time delay values over the pre-set time period; (iv)superimposing the spread of time delay values against other spreads oftime delay values of a plurality of other detected ultrasonic pulses orwaves; and (v) analysing the superimposed spreads of time delay valuesto determine if a proportion of the detected ultrasonic pulses or wavesare being detected at one or more particular time delay values after thetime of detection of the detected radio frequency pulses or waves.